Hi, I tried to fit the correction factor with first order polynomial from the second bin and on the last bin I get negative value: #mx_l mx_h corr err_corr 0.00 1.55 1.821 +- 0.577 1.55 1.90 3.260 +- 0.412 1.90 2.20 2.837 +- 0.304 2.20 2.50 2.446 +- 0.262 2.50 2.80 2.055 +- 0.301 2.80 3.10 1.664 +- 0.398 3.10 3.40 1.273 +- 0.521 3.40 3.70 0.882 +- 0.656 3.70 4.20 0.361 +- 0.844 4.20 5.00 -0.486 +- 1.160 (eps file attached) So how do I proceed on this? Antonio